Previous encapsulating polymers for water-based drilling fluids are not stable at high temperature. When exposed to high temperatures for a prolonged time, the polymers exhibit decreased viscosity. The decreased viscosity affects encapsulation ability, rheology, and the ability to work with fluid loss agents in the drilling fluid to decrease fluid loss.
The ability of a drilling fluid to carry cuttings increases as viscosity increases. If the drilling fluid is not viscous enough, the cuttings will be brought to the surface. If the drilling fluid is not viscous enough, it may migrate into the formation. Correct viscosity assists in forming a wall cake by creating a film of particles and sealing the wall. A drilling fluid should not be too viscous because that will prevent the cuttings from settling out in the mud pits.
Previously known encapsulating polymers for water-based drilling fluids were not stable at high temperatures. After aging at a high temperature (>300° F.) for an extended period (16 hours), the viscosity of the polymer solution decreases significantly due to polymer degradation. Less viscous fluids flow easier and the loss of viscosity had several impacts on the properties of the drilling fluid. First, the fluids partially lose their inhibitive and encapsulating capacity. Second, the viscosity change affects the rheology of the drilling fluid. In addition, the fluid loss characteristic of the drilling fluid could change as the encapsulating polymers often work synergistically with the fluid loss agents.
Therefore, it would be advantageous to have a polymer for use in water-based drilling fluids that does not significantly lose viscosity at high temperatures over time.